One human variety, first appearing about 2 million years ago, has provided a fossil skeleton even more complete than Lucy's.

It is of a boy, about eleven years old and not yet fully grown, who once stood slim and upright with a height of about 1.62m (5'4'). His fossil, discovered in Kenya in 1985, has been dated to about 1.6 million years ago - some 400,000 years after the earliest known remains of his species.

Humans of this kind have been named Homo erectus ('upright man'). With them we reach the first creatures to be undeniably our ancestors and ours alone.

Argument will long continue as to which of the earlier species of Homo habilis (and before them of Australopithecus) are the link between us and the origins of life. But there is little doubt about Homo erectus, because by about a million years ago he is the only surviving hominid. All other man-like creatures have died out -- presumably because they find it impossible to compete for the necessities which they (but not the apes) require in common with Homo erectus.

Out of Africa: more than a million years ago

Homo erectus is the variety of human who moves out of the continent of Africa, to spread through much of Asia and Europe. This move from Africa is usually dated to about a million years ago, but this may be too recent. First reports of two skulls found in 1999 at Dmanisi, in South Georgia, describe them as 1.8 million years old.

Fossil remains of this kind have been found as far afield as Java in southeast Asia (the first to be discovered, in 1891), Beijing in northern China, and within Europe in Greece, Germany and England - in addition to numerous sites in Africa. The European skulls differ from the Asian in various ways (larger brains, smaller teeth), causing some anthropologists to classify them not as Homo erectus but as an archaic version of our own species, Homo sapiens.

While it is certain that we all descend from Homo erectus, precisely how we do so is as much disputed as any other topic in this thorny field of human origins.

One theory is that after moving out of Africa around a million years ago Homo erectus evolves towards modern man in different ways in different parts of the globe, but that the regional groups retain enough contact and sufficient interbreeding to prevent any of them evolving into a separate species.

A rival theory involves two waves of exodus from Africa. The first, it is agreed, is that of Homo erectus more than a million years ago. But by this theory Homo erectus then evolves in different parts of the world into increasingly distinct varieties of human.

The variety of human evolving during this period in Africa, according to this theory, is modern man - descending from the original Homo erectus stock.

Modern man, this theory continues, comes out of Africa in a second exodus -- perhaps about 150,000 years ago -- and subsequently becomes the only human species on earth. Other humans known to us during that period (most notably the Neanderthal man of Eurasia) are therefore different varieties evolving locally. They are all eventually replaced by modern man, in a slow colonial expansion from Africa through Asia and, eventually, into Europe.

DNA analysis of different ethnic groups has provided evidence which seems to support this second theory, for it suggests that Africa has had a stable human population longer than any other part of the globe.

Another discovery supporting the 'out of Africa' theory is the date given by new technology to human remains found in the 1930s in caves at Skhul and Qafzeh, in modern Israel. They are of anatomically modern people, similar to ourselves, but they are now known to have lived about 90,000 years ago.

Modern humans spread through Asia (and even reach Australia) by 50,000 years ago, and appear in western Europe 35,000 years ago. The people of Skhul and Qafzeh live, much earlier, in the first habitable region on the land route from Africa. This does seem to cast them as ancestral figures on a long and slow second journey out of Africa.

There are certain undisputed facts which each of the rival theories must accomodate.

We have fossils of several varieties of human from within the last million years. All ultimately derive from the same ancestor, Homo erectus. These fossil varieties differ in many of their characteristics. Yet we ourselves are now the only surving human species. To accomodate these facts, there can only be two possible explanations - regardless of which parts of the planet are involved.

Either we have evolved from several of the various descendants of Homo erectus (among which Neanderthal man is the best known), while nevertheless remaining one species. Or our particular branch of Homo erectus has replaced all the others.

The possibility that our species replaced all other humans sounds dramatic, but need not be so. On an evolutionary scale replacement is not necessarily a scene of battle and carnage.

In Britain, for example, grey squirrels are steadily driving the smaller red squirrels northwards, because the grey variety is more adaptable in its use of the available woodland. The process is likely to lead, in a relatively short time, to the replacement of one species by the other.

But while it happens there is nothing cataclysmic about it.

The use of tools

It is a commonplace that humans are distinguished from other creatures by a technological ability, and man has often been described as a tool-using animal. The distinction is not entirely valid. Some animals do use tools. Chimpanzees are the most often quoted example, stripping a twig to plunge it into an anthill and then eating the tasty termites which cling to the end of it.

A more modern example of tool-using is that of crows living in a walnut avenue in the Japanese town of Sendai. The walnuts are too hard to crack. So the crows have taken to dropping them on a pedestrian crossing where they are crushed by the passing traffic. When it is the pedestrians' turn, the crows fly in to bear off the fragments.

But there is a difference between using a tool which comes to hand, however improbably, and fashioning one for a purpose. Shaping a tool for cutting or scraping (two basic and useful functions) is a difficult task. Such a tool must be made of a hard material, and the hardest material easily available on the surface of the earth is stone. But how does one shape a stone without tools?

The history of human technology begins with the discovery of how to give stone a cutting edge. The type of stone found most suitable for the purpose is flint.